Fluid-actuated torque transfer mechanisms or clutches are hydraulically-actuated devices that selectively connect a pair of rotating shafts, such as an input shaft and an output shaft of an automotive transmission. An application or engagement of one or more clutches connects the two shafts so that torque from the input shaft, which can be connectable to and driven by an internal combustion engine, a battery, or another suitable energy source, is smoothly transferred to the output shaft. In order to shift gears within the transmission, an off-going clutch is disengaged while an on-coming clutch is engaged. Fluid clutches generally provide a relatively rapid response time, as well as a smooth and efficient operation.
In order to engage a fluid clutch, an apply piston is moved in a particular manner using a controlled supply of pressurized fluid. The fluid enters an apply chamber, which is separated from a return or balance chamber by the apply piston. In order to effectively transfer torque across the fluid clutch, the apply chamber must first be filled with pressurized fluid prior to clutch engagement, a process commonly referred to as a clutch fill event. Various methods and devices exist for determining when an apply chamber is sufficiently filled with hydraulic fluid. For example, one or more transducers or pressure sensors can measure a fluid pressure within the apply chamber, and a pressure switch detect clutch fill and therefore control a valve or other fluid control mechanism which supplied the fluid to the apply chamber. However, precise measurement of a dynamically changing fluid pressure and/or remaining volume of an apply chamber can be less optimal, due in part to the potential of direct measurement devices or sensors to introduce feedback error over time and cost-related issues.